We are attempting to further characterize the phenotypic expression of MSV defectiveness in the heterologous human host S plus L minus cells. We are continuing to attempt to demonstrate a viral etiology in pediatric solid tumors. Certain aspects of the tissue culture system using human cell lines susceptible to MSV transformation are being studied in order to improve the chances for detection of human sarcoma viruses. Rapid tissue culture assays are being developed using human S plus L minus cells for possible detection of replicating nontransforming human type C viruses.